Phenylketonuria (PKU) is a disorder usually diagnosed by newborn screening. Persons with PKU cannot properly process phyenylalanine (phe), an amino acid found in food. The only known treatment for PKU is a diet consisting of a protein source without Phe called a Medical Food (formula) plus limited amounts of low protein foods. The diet is started immediately after PKU is diagnosed to prevent mental retardation in newborns. In the past, many persons with PKU were taken off the diet around school age. Some of them stopped the diet on their own. Researchers found that the children off diet began losing IQ points. The current recommendation is for all persons with PKU to remain on the Medical Food and protein restricted diet throughout life. Of special concern is the effect of PKU on pregnancy. Women with PKU who were not on the diet before and during pregnancy delivered babies with serious birth defects. The Maternal PKU Study began collecting information from women with PKU in 1984 to determine the effects of the phe-restricted diet and Medical Food before and during pregnancy. The purpose of the study is to determine the proper blood phe levels needed for the woman to have the best chance of having a healthy baby. The study checks the dietary intake of the women and their blood phe levels and other nutrients in the blood such as, iron and protein. The children born to women in the study receive a thorough physical at birth and yearly thereafter. Developmental testing is done on each child at 1, 2, 4, 7 and 10 years of age. The mother's blood phe levels and diet before and during pregnancy are compared to the child's growth and development. From this information the study aims to develop guidelines for clinics caring for women with PKU. The project has significant implications for preventing birth defects in the babies born to women with PKU.